Coupled cavity travelling wave tubes

ABSTRACT

A travelling wave tube having a coupled cavity slow wave structure is provided with velocity tapering to slow the speed of travel of an applied radio frequency wave at the output end of the tube so that it is maintained substantially in step with the decreasing velocity of the electron beam. Such velocity tapering is achieved in the present invention by varying the resonant frequency of the coupling elements coupling adjacent cavities of the slow wave structure in accordance with a predetermined tapering law so that the resonant frequency of the coupling elements towards the output end of the tube approaches the resonant frequency of the cavities.

United States Patent [191 King et al. 1

[451 Nov. 5, 1974 COUPLED CAVITY TRAVELLING WAVE TUBES [75] Inventors: Robin Charles Moorhouse King,

Chelmsford, England; Neville Wreiord Harris, Acton, Mass.

[73] Assignee: English Electric Valve Company Limited, Chelmsford, England 221 Filed: Feb. 22, 1973 21 Appl.No.:334,939

[52] U.S. Cl. 3l5/3.6, 315/35 [51] Int. Cl. H01] 5/34 [58] Field of Search 315/3.5, 3.6

[56] References Cited UNITED STATES PATENTS 3,274,428 9/1966 Harris 3l5/3.6

3,374,390 3/1968 Ruetz et a1 BIS/3.6 3,448,326 6/1969 Arfin et a1. 315/35 3,532,924 10/1970 Roumbanis..... 315/35 3,555,462 1/1971 Horigome 315/35 6/1972 Butwell 3l5/3.5 6/1972 James ..315/3.5

Primary Examiner-Archie R. Borchelt Assistant Examiner-Saxfield Chatmon, Jr.

A ttorrzey, Agent, or Firm-Baldwin, Wight & Brown 5 7] ABSTRACT A travelling wave tube having a coupled cavity slow wave structure is provided with velocity tapering to slow the speed of travel of an applied radio frequency 9 wave at the output end of the tube so that it is maintained substantially in step with the decreasing velocity of the electron beam. Such velocity tapering is achieved in the present invention by varying the resonant frequency of the coupling elements coupling adjacent cavities of the slow wave structure in accordance with a predetermined tapering law so that the resonant frequency of the coupling elements towards the output end of the tube approaches the resonant frequency of the cavities.

10 Claims, 6 Drawing Figures COUPLED CAVITY TRAVELLING WAVE TUBES This invention relates to travelling wave tubes and in As is well known a travelling wave tube includes a slow wave structure positioned to be adjacent an elec- .tron beam and in known so-called coupled cavity travelling wave tubes this slow wave structure comprises a series of coupled cavities with coupling elements (e.g., slots) between adjacent cavities and with a central aperture through the cavities through which the electron beam passes from the electron gun to a collector electrode at the output end of the tube. The basic operation of such tubes and the manner in which the slow wave structure slows down an applied r.f. wave to the speed of the electron beam so as to allow interaction therebetween is well known and will not be described herein. It is also known that as the electron beam approaches the collector electrode its velocity is reduced due to the overall transfer of kinetic energy from the beam electrons to the circuit fields of the cavity structure and that it is at the output end of the tube that the majority of power transfer from the electron beam to the circuit fields takes place. At the output end therefore the slow wave structure needs to be adapted to reduce the radio frequency (r.f.) wave speed even further so as to correspond to the reduced speed of the electron beam. One known expedient with such travelling wave tubes is to employ what is known as velocity tapering at the output end. The normal method of achieving velocity tapering is to gradually reduce the height of the cavities (i.e., length in the beam direction) over the last few cells at the output end of the tube so that even though the electron beam loses energy to the r.f. circuit wave its axial velocity keeps substantially in step with that of the r.f. wave.

However the aforementioned method of providing a velocity taper reduces the r.f. circuit wave velocity by substantially the same amount over the whole operating band of frequencies which limits the useful bandwidth since due to the dispersive nature of coupled cavity structures the electron beam is slowed more at the low frequency end of the operating band than at the high frequency end of the operating band.

The present invention seeks to provide an improved coupled cavity travelling wave tube in which the velocity tapering is frequency dependent so as to slow the circuit r.f. wave at low frequencies relatively more than at high frequencies in the operating band and so increase the useful bandwidth of the tube.

According to this invention a travelling wave tube includes an electron gun for producing an electron beam; a collector electrode for collecting said electron beam; a coupled cavity slow wave structure between said gun and collector for coupling radio frequency energy into and out of said electron beam; a radio frequency signal input coupled to said slow wave structure to feed thereto. radio frequency signals to be coupled to the beam; and a radio frequency signal output coupled to the slow wave structure for extracting amplified radio frequency signals therefrom; and in which tube adjacent cavities of the slow wave structure are coupled one to the other by resonant coupling elements, the resonant frequencies of some of which elements at the output end'of the structure correspond with a predetermined tapering law in which the frequency approaches the cavity resonant frequency towards the output end.

Preferably said predetermined tapering law is a linear law such that the difference in resonant frequencies between successive elements of said some elements is the same. The resonant elements may be slots in common partition walls between the cavities which slots forming said some elements vary in length from one partition wall to the next towards the output end so as to change their resonant frequency.

In slot coupled structures of the clover leaf type with radial slots and of the harmonic interaction coupled cavity structure kind with circumferential slots the slot length is increased to decreasae the resonant frequency. ln slot-coupled structures of the inverted or long slot kind the slot length is decreased to increase the resonant frequency.

In a further preferred coupled cavity structure the cavities are loop coupled, by a structure such as a centipede structure known per se in which the coupling elements are 8" shaped loops in common partition walls between the cavities and in which either the circumferential spacing between the loops is decreased or the loop amplitude is increased to provide a reduction in the resonant frequency of the elements.

In the above described embodiments the variation of the resonant frequency of said elements is used to provide velocity tapering" but it may be used to provide only a part of the total required velocity taper the remaining part being provided by variation of the cavity height at the output end.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which,

FIG. 1 shows a longitudinal cross-section of a clover leaf type coupled cavity travelling wave tube in accordance with this invention.

FIG. 2a is a transverse section taken along section line AA in FIG. 1.

FIG. 2b is a transverse section taken along section line 8-8 in FIG. 1.

FIG. 2c is a transverse section of a centipede type structure according to this invention.

FIG. 2d is a longitudinal section of the centipede structure.

FIG. 3 is an exemplary graphical sketch.

In the figures like numbers denote like parts.

Referring to FIGS. 1 and 2a and 2b, the clover leaf type coupled cavity travelling wave tube shown therein has an electron gun I for providing an electron beam, a wave retarding structure 2 through which the electron beam is passed, and a collector electrode 3 for gathering the electrons of the beam. The wave retarding structure 2 is formed in three portions 4,5,6, portions 4 and 5 being negatively coupled clover leaf structures, which form transmission lines having band-pass characteristics, separated by a drift section 6. The portions 4 and 5 are both formed from a succession of resonant cavities 7 (only three of which are shown in each portion 4, 5 for clarity although in practice it is likely that between 10 and 20 sections would be used in each portion) separated from one another by a common interconnecting or partition wall 8 in which is a centrally disposed circular hole 9, through which the electron beam passes, and eight radial resonant coupling elements in the form of slots 10. Each cavity 7 is provided with four inwardly projecting noses 11 which are circumferentially displaced by 1r/4 radians from one cavity to the next. The resonant cavity 7 nearest to the electron gun I has an opening to accept an input waveguide 12 and the resonant cavity 7 nearest to the collector electrode 3 is provided with an output waveguide 13 which is terminated in a load (not shown). The resonant cavities 7 on either side of the drift section 6 are also provided with waveguides 14 which are terminated in loads (not shown). Surrounding the resonant cavities 7 is a focussing coil 15 which provides a magnetic field to focus and confine the electron beam in a manner known per se. A power supply 16 has its negative terminal connected to the electron gun I and its positive (earth) terminal connected to the resonant cavities 7. The collector electrode 3, which is insulated from the slow wave structure 2 (by means not shown), is connected to the positive terminal of a further power supply 17 whose negative terminal is connected to the electron gun I. The power supply 17 is of a lower voltage than supply 16 and connected in this manner the travelling wave tube is capable of what is known as depressed collector operation.

In operation, the cavities 7 are all arranged to resonate at substantially the same frequency and r.f. energy is introduced into the cavities 7 via input waveguide 12. The r.f. energy modulates the electron beam in the portion 4 of the slow wave structure and is extracted from waveguide 14 nearest to the electron gun I. The modulated beam passes through the drift section into portion where it interacts with cavities 7 to provide an amplified signal which is removed from output waveguide 13. Waveguide l4 nearest the collector electrode 3 is provided to absorb any unwanted reflected energy.

As so far described the travelling wave tube is a known tube without velocity tapering in which the velocity of the electron beam towards the output end is lower than the axial velocity of the r.f. wave. The present invention provides velocity tapering by shaping and dimensioning the slots over the portion 5 so as to maintain the r.f. wave substantially in synchronism with the electron beam. This is achieved by linearly decreasing the resonant frequency of the slots 10 from the drift section 6 towards the collector electrode 3 by increasing their length such that the resonant frequency of the slots 10 approaches the resonant frequency of the cavities 7.

Referring to FIG. 3, the characteristics shown therein relate the phase change of the r.f. wave with the frequency of the r.f. wave for a cavity 7 prior to the commencement of the slot length tapering and for a cavity 7 at the output end of the travelling wave tube, i.e., at the completion of slot length tapering. In this respect the solid line characteristic represents the cavity 7 prior to the commencement of slot length tapering and the chain dotted line characteristic represents the cavity at the completion of slot length tapering drawn for an abscissae of phase change/cavity (BL) and ordinates of frequency (f) of the r.f. wave. It will be seen from these curves, when comparing the two structures, that the change in (BL) and hence the reduction in the circuit wave velocity is greater at a lower frequency (fl) in the r.f. wave band than at a higher frequency (f2). Therefore by the expedient of varying the length of the slots 10 the resulting velocity taper, represented by (,BL), has the effect of increasing the operating band width of the travelling wave tube by maintaining substantially the required electron beam to r.f. wave relationship over a greater part of the operating band than with previously known coupled cavity travelling wave tubes.

The present invention has been used with particular advantage in a depressed collector travelling wave tube, connected as shown in FIG. 1 and with power supply 17 maintaining the collector electrode 3 at a negative potential with respect to the cavities 7, in a known manner, to produce a decelerating electric field. It is well known that electrons in a beam, for any one particular frequency, do not all travel at the same velocity with the result that if the collector electrode potential is arranged to gather electrons having median and higher velocities, then slower electrons are turned round by the decelerating electric field and travel backwards towards the electron gun 1. Also it has been found that the number of electrons turned round by the decelerating field varies with frequency. However,

using the frequency dependent velocity taper of the present invention, the taper may be arranged so that the slowing down of the r.f. wave more closely matches the retardation of the electron beam over the r.f. wave pass band with the result that the electron beam velocity speed is considerably less variant with frequency.

It will be realised by those skilled in the art that the present invention is applicable to any known coupled cavity retarding structure by changing the resonant frequency of the coupling element from one cavity to the next, thereby producing a frequency dependent phase change in adjacent cavities. Thus in the harmonic interaction coupled cavity structure where the coupling elements are circumferentially disposed slots, the slots are arranged to increase in length from one partition wall to the next in a direction towards the collector electrode as shown in FIG. 2d. In the inverted or long slot structure where the resonant frequency of the coupling slots is below that of the cavities as shown in FIG. 2e, the length of the coupling slots is decreased in length from one partition wall to the next as the r.f. wave approaches the travelling wave tube output.

A further coupled cavity structure to which the present invention relates is a loop coupled or centipede structure as shown in FIGS. 20 and 2d in which the coupling elements in the partition walls are S shaped loops. In such a structure the resonant frequency of the coupling loops may be varied either by decreasing the circumferential spacing between the loops or by increasing the amplitude of the loops from one partition wall to the next approaching the collector electrode.

Although the invention has so far been described in connection with a coupling element resonant frequency which has a linear taper decreasing towards the output of the travelling wave tube it is to be understood that other laws of taper may also be applied. Also it is to be realised that the present invention is not limited to the use of a travelling wave tube having a drift section but that tubes having a continuous coupled cavity structure or several drift sections may be employed in which case again only some of the elements (namely those separating cavities nearest the output end of the tube) will provide velocity tapering.

A combination of cell height taper as is already known and as is shown in FIG. 20 and frequency depen dent velocity taper may also be employed with advantage so that each kind of taper provides only a proportion of the overall desired velocity taper.

We claim:

1. A travelling wave tube including an electron gun for producing an electron beam; a collector electrode for collecting said electron beam; a coupled cavity slow wave structure between said gun and collector for coupling radio frequency energy into and out of said electron beam and having a pass band defining a predetermined center frequency; a radio frequency signal input adjacent said electron gun and coupled to said slow wave structure to feed thereto radio frequency signals to be coupled to the beam; and a radio frequency signal output adjacent said collector electrode and coupled to the slow wave structure for extracting amplified radio frequency signals therefrom; and in which tube adjacent cavities of the slow wave structure are coupled one to the other by resonant coupling elements, the resonant frequencies of which elements are different from said center frequency and the resonant frequencies of some of which elements at the output end of the structure change successively to approach said center frequency toward the radio frequency output end of the tube.

2. A travelling wave tube as claimed in claim 1 wherein said change of coupling elements resonant frequency is in accordance with a linear law such that the difference in resonant frequencies between successive elements of said some elements is the same.

3. A travelling wave tube as claimed in claim 1 wherein the resonant elements are slots in common partition walls between the cavities which slots forming said some elements vary in length from one partition wall to the next towards the output end so as successively to decrease their resonant frequency.

4. A travelling wave tube as claimed in claim 3 wherein the slot coupled structure is of the clover leaf type with radial slots and in which the slot length is increased to decrease the resonant frequency toward the output end of the tube.

5. A travelling wave tube as claimed in claim 3 wherein the slot coupled structure is of the harmonic interaction kind with circumferential slots in which the slot length is increased to decrease the resonant frequency toward the output end of the tube.

6. A travelling wave tube as claimed in claim 1 wherein the slot coupled structure is of the inverted kind so that the resonant frequencies of said elements are less than said center frequency and in which the slot length is decreased to increase the resonant frequency toward the output end of the tube.

.7. A travelling wave tube as claimed in claim 3 wherein the cavities are loop coupled, by a structure such as a centipede structure known per se in which the coupling elements are S shaped loops in common partition walls between the cavities and in which the circumferential spacing between the loops is decreased to provide a reduction in the resonant frequency of the elements.

8. A travelling wave tube as claimed in claim 3 wherein the cavities are loop coupled, by a structure such as a Centipede structure known per se in which the coupling elements are S shaped loops in common partition walls between the cavities and in which the loop amplitude is increased to provide a reduction in the resonant frequency of the elements.

9. A travelling wave tube as claimed in claim 3 wherein the resonant frequency of said elements is used to provide only a part of the total required velocity taper the remaining part being provided by variation of the cavity height at the output end.

10. A travelling wave tube as claimed in claim 1 wherein the height of the cavities is varied in accordance with a predetermined taper towards the output end of the tube. 

1. A travelling wave tube including an electron gun for producing an electron beam; a collector electrode for collecting said electron beam; a coupled cavity slow wave structure between said gun and collector for coupling radio frequency energy into and out of said electron beam and having a pass band defining a predetermined center frequency; a radio frequency signal input adjacent said electron gun and coupled to said slow wave structure to feed thereto radio frequency signals to be coupled to the beam; and a radio frequency signal output adjacent said collector electrode and coUpled to the slow wave structure for extracting amplified radio frequency signals therefrom; and in which tube adjacent cavities of the slow wave structure are coupled one to the other by resonant coupling elements, the resonant frequencies of which elements are different from said center frequency and the resonant frequencies of some of which elements at the output end of the structure change successively to approach said center frequency toward the radio frequency output end of the tube.
 2. A travelling wave tube as claimed in claim 1 wherein said change of coupling elements'' resonant frequency is in accordance with a linear law such that the difference in resonant frequencies between successive elements of said some elements is the same.
 3. A travelling wave tube as claimed in claim 1 wherein the resonant elements are slots in common partition walls between the cavities which slots forming said some elements vary in length from one partition wall to the next towards the output end so as successively to decrease their resonant frequency.
 4. A travelling wave tube as claimed in claim 3 wherein the slot coupled structure is of the ''''clover leaf'''' type with radial slots and in which the slot length is increased to decrease the resonant frequency toward the output end of the tube.
 5. A travelling wave tube as claimed in claim 3 wherein the slot coupled structure is of the harmonic interaction kind with circumferential slots in which the slot length is increased to decrease the resonant frequency toward the output end of the tube.
 6. A travelling wave tube as claimed in claim 1 wherein the slot coupled structure is of the inverted kind so that the resonant frequencies of said elements are less than said center frequency and in which the slot length is decreased to increase the resonant frequency toward the output end of the tube.
 7. A travelling wave tube as claimed in claim 3 wherein the cavities are loop coupled, by a structure such as a ''''centipede'''' structure known per se in which the coupling elements are ''''S'''' shaped loops in common partition walls between the cavities and in which the circumferential spacing between the loops is decreased to provide a reduction in the resonant frequency of the elements.
 8. A travelling wave tube as claimed in claim 3 wherein the cavities are loop coupled, by a structure such as a ''''centipede'''' structure known per se in which the coupling elements are ''''S'''' shaped loops in common partition walls between the cavities and in which the loop amplitude is increased to provide a reduction in the resonant frequency of the elements.
 9. A travelling wave tube as claimed in claim 3 wherein the resonant frequency of said elements is used to provide only a part of the total required ''''velocity taper'''' the remaining part being provided by variation of the cavity height at the output end.
 10. A travelling wave tube as claimed in claim 1 wherein the height of the cavities is varied in accordance with a predetermined taper towards the output end of the tube. 